The present invention relates to chromatic dispersion and polarization mode dispersion compensation, and more particularly to chromatic dispersion and polarization mode dispersion compensation accumulated in a wavelength division multiplexed optical fiber transmission line.
Fiber optic networks are becoming increasingly popular for data transmission because of their high speed and high capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this specification, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d
One common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect wherein the channels comprising an optical signal travel through an optic fiber at different speeds, e.g., longer wavelengths travel faster than shorter wavelengths. This is a particular problem that becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of the signal. Therefore, for a fiber optic communication system to provide a high transmission capacity, the system must compensate for chromatic dispersion. The exact value of the chromatic dispersion produced in a channel of a wavelength-division multiplexed fiber optic communications system depends upon several factors, including the type of fiber and the wavelength of the channel.
For dense wavelength division multiplexer (DWDM) systems or for WDM or DWDM systems with a wide wavelength spacing between the shortest and longest wavelength channels, the common approach is to allow chromatic dispersion to accumulate within spans of fiber and to compensate for dispersion at the ends of spans through the use of in-line dispersion compensator apparatuses.
A second common and well-known problem in the transmission of optical signals is polarization mode dispersion (PMD). PMD is the phenomenon by which differently polarized components, or sub-signals, comprising an optical signal propagate with different speeds or, alternatively, propagate along differing-length optical paths. This duality of speeds or paths can also cause unacceptable broadening of the digital pulses comprising a signal that increases in severity with increasing transmission speed. The maximum acceptable PMD-induced optical path length difference is the cumulative result of all PMD effects in all the optical elements through which a signal propagates, including fiber and non-fiber optical components. Although the PMD broadening of optical fiber increases as the square root of fiber length, the PMD broadening caused by birefringent components is linearly related to the cumulative optical path difference of all such components.
The chromatic dispersion characteristics of optical fibers are not constant but depend upon wavelength, as illustrated in FIG. 1, which presents graphs of Group Velocity Dispersion, D, against wavelength, for typical examples of three commonly used fiber types. In FIG. 1, the quantity D (psxe2x88x92kmxe2x88x921xe2x88x92nmxe2x88x921) is defined by the relationship of Eq. 1                     D        =                                                            ⅆ                                  xe2x80x83                                                            ⅆ                λ                                      ⁢                          xe2x80x83                        ⁢                          (                              1                                  v                  g                                            )                                =                                    1              L                        ⁢                          xe2x80x83                        ⁢                          (                                                ⅆ                                      τ                    g                                                                    ⅆ                  λ                                            )                                                          (        1        )            
in which xcex is the channel wavelength (nm), vg is the group velocity (km/ps), xcfx84g is the group delay time (ps), and L is the fiber length (km). If vg decreases with increasing wavelength (i.e., longer or xe2x80x9credxe2x80x9d wavelengths travel slower than relatively shorter or xe2x80x9cbluexe2x80x9d wavelengths) then D is positive, otherwise D is negative. Because all three fiber types illustrated in FIG. 1 are deployed in telecommunications systems, the requirements for dispersion compensators vary widely.
Conventional apparatuses for dispersion compensation include dispersion compensation fiber, chirped fiber Bragg gratings coupled to optical circulators, and conventional diffraction gratings disposed as sequential pairs.
A dispersion compensation fiber, which is used in-line within a fiber communications system, has a special cross-section index profile so as to provide chromatic dispersion that is opposite to that of ordinary fiber within the system. The summation of the two opposite types of dispersion negates the chromatic dispersion of the system. However, dispersion compensation fiber is expensive to manufacture, has a relatively large optical attenuation, and must be relatively long to sufficiently compensate for chromatic dispersion.
A chirped fiber Bragg grating is a special fiber with spatially modulated refractive index that is designed so that longer (shorter) wavelength components are reflected at a farther distance along the chirped fiber Bragg grating than are the shorter (longer) wavelength components. A chirped fiber Bragg grating of this sort is generally coupled to a fiber communications system through an optical circulator. By causing certain wavelength components to travel longer distances than other wavelength components, a controlled delay is added to those components and opposite dispersion can be added to a pulse. However, a chirped fiber Bragg grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped fiber Bragg gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system. Furthermore, fiber Bragg gratings generally do not compensate PMD.
A conventional diffraction grating has the property of outputting different wavelengths at different angles. By using a pair of gratings in a coupled spatial arrangement, this property can be used to compensate chromatic dispersion in a fiber communications system. In such a spatial grating pair arrangement, lights of different wavelengths are diffracted from a first grating at different angles. These lights are then input to a second grating that diffracts them a second time so as to set their pathways parallel to one another. Because the different lights travel with different angles between the two gratings, certain wavelength components are made to travel longer distances than other wavelength components. Chromatic dispersion is produced in the spatial grating pair arrangement because the wavelength components that travel the longer distances incur time delays relative to those that travel the shorter distances. This grating-produced chromatic dispersion can be made to be opposite to that of the fiber communications system, thereby compensating the chromatic dispersion within the system. However, a practical spatial grating pair arrangement cannot provide a large enough dispersion to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. More specifically, the angular dispersion produced by a diffraction grating is usually extremely small, and is typically approximately 0.05 degrees/nm. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, the two gratings of a spatial grating pair would have to be separated by a very large distance, thereby making such a spatial grating pair arrangement impractical.
Accordingly, there exists a need for an improved chromatic dispersion and polarization mode dispersion compensator. The improved compensator should produce an adjustable chromatic dispersion while simultaneously compensating for polarization mode dispersion. The present invention addresses such a need.
The present invention provides a chromatic dispersion and polarization mode dispersion compensator utilizing a Virtually Imaged Phased Array (VIPA) and birefringent wedges to moderate chromatic dispersion and polarization mode dispersion (PMD). The compensator in accordance with the present invention propagates the composite optical signal in a forward direction; separates the wavelengths in the band of wavelengths in each of the plurality of channels, where the each of the wavelengths in the band is spatially distinguishable from the other wavelengths in the band; spatially separates each wavelength of each band of wavelengths into a plurality of polarized rays; and reflects the plurality of polarized rays toward a return direction, where dispersion is added to the reflected plurality of polarized rays such that the unwanted chromatic dispersion and PMD are compensated. The compensator in accordance with the present invention provides simultaneous tunable compensation of chromatic dispersion and polarization mode dispersion utilizing a single apparatus. A system which utilizes the compensator is thus cost effective to manufacture.